With the development of mobile communications, multiband base station antenna arrays are required to simultaneously support multiband and multi-standard wireless systems. It is common to use sub-arrays operating at different bands to realize a dual- or multiband array when the operating frequency bands are far from each other, i.e., 820-960 and 1710-2170 MHz. However, due to serious mutual coupling between the sub-arrays, the method becomes inapplicable for designing arrays with very close frequency bands, such as DCS (1710-1880 MHz) and WCDMA (1920-2170 MHz) bands. Although increasing the su-barray separation can effectively reduce mutual coupling, the size of the array is getting bulky.
To solve this problem, two typical methods have been employed. One approach is to use a diplexer in cascade with a full-band antenna array to realize dual-band performance and compact size. Satisfying port isolation can be achieved by utilizing a diplexer. However, the insertion loss introduced by the diplexer becomes serious when the two operating bands are very close and antenna gain will be degraded. Moreover, it is difficult to individually control the downtilt of each band using only one antenna array, which cannot meet the requirements of wireless network optimization. The other approach is to use two full-band sub-arrays placed side by side with the same polarization direction. A decoupling feed network is necessary to be implemented to improve the port-to-port isolation and other additional decoupling networks have also been investigated, such as electromagnetic band-gap (EBG) structures, defected ground plane structures, band-stop decoupling units. It was found that these methods also affect the radiation performance of the antenna, such as radiation efficiency, front-to-back ratio, and antenna gain. The problem will be much toil less if the out-of-band radiation of two sub-array elements can be suppressed, and the mutual coupling will then be reduced at the same time.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.